You are rising some really very interesting points Bruton
As for what caused the stampede we can only guess that perhaps it was just the scent of the food which the Gang had sensed before as well combined with hunger and the thought that "whoever gets there last will get the least".
A modification of that last thought may also be the reason why there was no attempt of the Gang to join that herd. Perhaps the gang had no hope at all to get any help or protection from the herd in which it seemed to be mostly the everyoneone for him or herself principle. Apart from the dislike of any herd member to share foot with five more mouth's / beaks (no matter how small said mouths / beaks are) the gang might have been seen as just a delaying factor.
It is just a speculation and a weak one given that the fact that the herd does leave some food behind.
Perhaps other members of the Gang (Cera in case of LBT as we know it) might have objected to attempts to join a longneck herd in addition to the probable unwillingness of the herd to take them (with the possible exception of Littlefoot) along.
To me the whole scene appears a bit like it would have very well fit in if the
oasis scene had preceeded it.
I got to admit I never thought much about this scene when indeed it gives a lot of room for thoughts and speculations.